The shock adjuster rod, piston and needle valve are all removed from the shaft.
The parts are all laid out here. As you can see the piston valve uses slots and orifices in the valve body in conjunction with spring steel shims stacked up just like the foot valve. The piston valve primarily controls rebound damping and the needle valve allows fluid to bypass the piston valve. Adjusting the needle position via the rod and adjusting knob allows for external adjustment of the rebound damping.
The BOM is carefully checked to see what the differences between the new and old spec of valving are.
This is the old needle valve and adjusting knob. The knob spins the shaft which moves the needle valve up and down allowing it to meter the piston fluid bypass.
The new advanced needle valve is on the left and the old spec is on the right. As you can see the major difference between the two is that the newer valve is much bigger, over twice the size. This allows the new valve to bypass more fluid. This will particularly influence in reducing low speed rebound although it will also have a decent affect on high speed rebound as well. The rebound damping will increase quickly as the needle gets close to the seat. This should make the street ride a lot more comfortable but it will reduce fine control of body motion and will make less adjustment leeway as the suspension is adjusted stiffer. This will make finding the right set up for maximum track performance, particularly with sticky tires a little more difficult, a decent compromise if your car is not a race car. This sort of damping curve can have some advantages in the back of a front engine rear drive car.
This small hole is where the bypass oil flows out of. The size is fine for the stock needle valve but it is too small for the increased flow of the advanced needle valve.